In general terms, an optical-fibre transmission cable can have very varied structures, but a structure which is fairly well known may be referred to as an illustration.
Such a cable consists of a cylindrical support or ring, recessed on its periphery with longitudinal or helical grooves, in which the optical fibres are accommodated.
The cylindrical support has, along its axis, a reinforcing element which ensures the essential mechanical strength of the cable.
The assembly as a whole is, of course, accommodated in a sheath which ensures that the optical fibres are protected.
In some cases, the reinforcing elements can be arranged inside this sheath in addition to or instead of the axial reinforcing element.
These reinforcing elements are required to increase the tensile and compressive strength of the various elements forming the cable, to prevent the shrinkage and relaxation of the elements of the cable, and to allow them to withstand the considerable pulls which can occur, especially during the laying of the cables.
Where optical cables are concerned, it very quickly proved useful to be able to use non-metallic reinforcing elements. These non-metallic reinforcing elements are usually composed of glass or kevlar threads previously braided or otherwise and impregnated with a polymerizable resin; these threads are often obtained by means of a pultrusion process.
Products using thermoplastics are being studied at the present time, but the results are still unsatisfactory.
These products, although they usually behave well or moderately well under tension and compression, at all events sufficiently for most uses, nevertheless have a major disadvantage, namely their production cost. At the present time, this very high production cost makes it very difficult to use them in products such as opticalfibre cables. In fact, since the size of these reinforcing threads usually varies between 500 microns and 2 mm at most, to obtain a high uniformity of the assembly as a whole, very important precautions have to be taken at the moment when the threads are amalgamated with the polymerizable resin, thus necessitating extremely low production speeds (approximately 2 m/min). This low production speed alone is the reason for the very high cost of these reinforcing elements. In fact, it can be estimated that the "material" cost is scarcely a tenth of the "labor/machine" cost of such a product.
As regards the processes still being studied, which use thermoplastic resins, another difficulty is encountered, namely that of binding the various components to obtain a product which has good mechanical characteristics, particularly as regards bending, tension and compression.